Comprehensive Study Notes on Mitochondrial Biology and Genetics

Introduction to Mitochondria and Cellular Biology

• Definition of Mitochondria: Double-membrane organelles present in nearly all eukaryotic cells. They serve as central hubs for energy production, metabolism, and cell survival.

• Physiological Importance:     - ATP Generation: Mitochondria generate most cellular ATP via oxidative phosphorylation.     - Signaling Integration: They integrate metabolic, redox, calcium, and apoptotic signals.     - Tissue Essentiality: Vital for tissues with high energy demand, such as muscle, neurons, and the heart.

• Endosymbiotic Origin: Mitochondria are derived from an ancestral $\alpha$-proteobacterium engulfed by early eukaryotic cells. Evidence includes:     - Circular DNA (mtDNA).     - Bacterial-type ribosomes.     - Doubling membrane structure.     - Presence of cardiolipin in the inner membrane.

• Key Concept: Mitochondria are multifunctional organelles essential for life, metabolism, and cell fate, far exceeding the simple "powerhouse" label.

General Organization of the Mitochondrion

• Structural Regions:     - Outer Mitochondrial Membrane (OM): Contains porins; freely permeable to small molecules. Similar to bacterial outer membranes.     - Intermembrane Space (IMS): Composition similar to cytosol; contains apoptotic factors such as cytochrome c and AIF (Apoptosis Inducing Factor).     - Inner Mitochondrial Membrane (IM): Highly impermeable and rich in cardiolipin. Densely packed with proteins (ETC complexes, ATP synthase). Divided into two domains: the inner boundary membrane and the cristae membrane.     - Matrix: Enclosed by the inner membrane; contains enzymes for the TCA cycle, $\beta$-oxidation, and mtDNA replication. It houses mitochondrial ribosomes and nucleoids.

• Cristae: Infoldings of the inner membrane that increase surface area for oxidative phosphorylation. They connect to the inner boundary membrane via cristae junctions.

• Key Concept: The structure of mitochondria is compartmentalized and intimately linked to metabolic and signaling functions.

The Outer Mitochondrial Membrane (OM)

• Structural Features:     - Porins (VDACs): Form aqueous channels allowing diffusion of ions and metabolites up to $\sim 5\,kDa$.     - Permeability: Relatively permeable compared to the highly restricted inner membrane.

• Import Machinery:     - TOM Complex: The "Translocase of the Outer Membrane." It is the first gateway for nuclear-encoded mitochondrial proteins. Key receptors include Tom20, Tom22, and Tom70.     - Transfer to TIM: Proteins passing through TOM are transferred to TIM complexes (Inner Membrane). Proteins with N-terminal presequences are routed to the matrix via mtHsp70 pulling or to the intermembrane space via lateral release.

• Fission Machinery Host: Acts as the recruitment site for DRP1 during mitochondrial division, interacting with FIS1 and other adaptor proteins.

• Functional Roles: Regulates metabolite exchange, coordinates protein import, and participates in mitochondrial fission and quality control.

The Inner Mitochondrial Membrane (IM)

• Structural Characteristics:     - Impermeability: Only specific transporters allow passage of metabolites, which is essential for maintaining the proton gradient.     - Protein Density: Approximately $70\%$ protein content; houses all Electron Transport Chain (ETC) components and ATP synthase complexes.     - Cardiolipin Enrichment: A signature lipid that stabilizes ETC complexes and supercomplexes (respirasomes) and provides resistance to oxidative damage.

• Functional Domains:     - Inner Boundary Membrane: Faces the outer membrane.     - Cristae Membrane: Invaginations containing the OXPHOS machinery.

• Functional Roles: Site of oxidative phosphorylation, proton pumping, and transport of ADP/ATP (via ANT), phosphate (via Pi carrier), and pyruvate (MPC). It is a core regulator of metabolic flux and apoptosis.

Cristae Architecture and Cristae Junctions

• Cristae Specialization: Highly folded structures containing high densities of ETC complexes (I–IV) and ATP synthase.

• Cristae Junctions: Narrow tubular connections linking cristae to the inner boundary membrane. They control compartmentalization and regulate the diffusion of metabolites, cytochrome c (crucial for apoptosis), and protons.

• Cristae Remodeling: Dynamic process regulated by OPA1 and the MICOS complex. Essential for adapting metabolic output and facilitating cytochrome c release during apoptosis.

The Mitochondrial Matrix

• Composition: Dense, gel-like consistency containing $2/3$ of total mitochondrial proteins. It houses enzymes, mtDNA, ribosomes, tRNA, granules, fibrils, and tubules.

• Enzymatic Systems: Contains enzymes for the citric acid cycle (TCA), $\beta$-oxidation of fatty acids, and the pyruvate dehydrogenase system.

• Chemical Inventory: Includes ATP, ADP, AMP, inorganic phosphate ($Pi$), NAD, NADP, Coenzyme A, and ions such as $K^+$, $Mg^{2+}$, and $Ca^{2+}$.

• TCA Cycle Outcomes: Produces $CO_2$, NADH, and FADH$_2$. The latter two donate electrons to the respiratory chain to form ATP and water.

Orthodox vs. Condensed Mitochondrial States

• Orthodox State: Low respiratory activity. Matrix is expanded/less dense; cristae are relaxed. Occurs when there is low ADP availability or low metabolic demand (resting cells).

• Condensed State: High respiratory activity (State 3 respiration). Matrix is electron-dense and compact; cristae are tight and highly organized. Occurs during high metabolic demand (e.g., in muscle or neurons).

• Functional Significance: These transitions are rapid and reversible indicators of the cell's energetic condition.

Mitochondrial Dynamics: Fusion and Fission

• Mitochondrial Fusion:     - Purpose: Mix contents, complement damaged mtDNA/proteins, and maintain a healthy, interconnected network.     - Outer Membrane Fusion: Mediated by Mitofusin 1 (MFN1) and Mitofusin 2 (MFN2). GTP-dependent. MFN2 also regulates ER–mitochondria contacts (MAMs) and calcium exchange.     - Inner Membrane Fusion: Controlled by OPA1 (a dynamin-like GTPase). Maintains cristae structure. OPA1 cleavage (Opa1-L to Opa1-S) regulates remodeling during apoptosis.

• Mitochondrial Fission:     - Purpose: Generate new mitochondria for cell division, remove damaged regions (mitophagy), and adapt to stress.     - Master Regulator: DRP1 (Dynamin-related protein 1). Recruit to OM by adaptors like FIS1, MFF, MiD49, and MiD51. Forms ring-like structures; GTP hydrolysis drives constriction.     - ER Role: ER tubules often mark the site of constriction at ER–mitochondria contact sites.

Mitophagy (PINK1/Parkin Pathway)

• Definition: Selective form of autophagy to remove damaged mitochondria, preventing ROS accumulation and metabolic failure.

• Healthy Mitochondria: PINK1 is imported and degraded by proteases (e.g., PARL).

• Damaged Mitochondria:     1. Loss of membrane potential ($\Delta \psi$) prevents PINK1 import; PINK1 accumulates on the OM.     2. PINK1 recruits and phosphorylates Parkin (cytosolic E3 ubiquitin ligase) and ubiquitin.     3. Parkin ubiquitinates OM proteins (e.g., MFN1/2, VDAC).     4. Autophagy receptors (OPTN, NDP52) bind polyubiquitin chains and recruit LC3-positive membranes.     5. The autophagosome engulfs the mitochondrion and fuses with a lysosome for degradation.

• Clinical Relevance: Defects in PINK1 or Parkin are linked to early-onset Parkinson’s disease.

Intracellular Transport and Network Shape

• Mitochondrial Reticulum: Mitochondria form dynamic networks rather than isolated units. Elongated networks signify high ATP demand; fragmented states suggest stress or mitophagy.

• Cytoskeletal Movement:     - Kinesin: Anterograde transport (toward cell periphery).     - Dynein: Retrograde transport (toward MTOC).     - Anchors: Miro and Milton (TRAK) connect mitochondria to motor complexes.

• Tissue Organization: Neurons require long-distance trafficking; cardiac muscle features a lattice-like arrangement; sperm utilize a mitochondrial sheath in the midpiece.

Pyruvate Oxidation and the TCA Cycle

• Pyruvate Oxidation: Pyruvate from glycolysis is converted to acetyl-CoA by the Pyruvate Dehydrogenase Complex (PDC) in the matrix. Requires Thiamine (Vit. B1), FAD, NAD$^+$, CoA, and Lipoic acid. Produces $1\,NADH$ and $1\,CO_2$ per pyruvate.

• TCA Cycle: Oxidizes acetyl-CoA to $CO_2$. Per cycle, it generates:     - $3\,NADH$     - $1\,FADH_2$     - $1\,GTP$ (ATP equivalent).

• Biosynthetic Roles: Provides precursors for amino acids, heme, lipids, and glucose (gluconeogenesis).

Fatty Acid $\beta$-Oxidation

• Location: Mitochondrial matrix.

• Transport: Long-chain fatty acids require the carnitine shuttle: CPT-I (OM), Translocase (IM), and CPT-II (Matrix).

• Process: Cyclic removal of 2-carbon units producing acetyl-CoA, $1\,NADH$, and $1\,FADH_2$ per cycle.

• Significance: Provides significantly more ATP than carbohydrates. Essential for fasting, heart/muscle energy, and ketogenesis.

• Regulation: Inhibited by malonyl-CoA.

The Electron Transport Chain (ETC)

• Location: Embedded in the IM.

• Complexes and Carriers:     - Complex I (NADH:ubiquinone oxidoreductase): Accepts electrons from NADH.     - Complex II (Succinate dehydrogenase): Accepts electrons from FADH$_2$; does not pump protons.     - Coenzyme Q (Ubiquinone): Lipid-soluble carrier; transfers electrons to Complex III.     - Complex III (Cytochrome bc1 complex): Pumps protons; passes electrons to cytochrome c.     - Cytochrome c: Mobile carrier in the IMS; transfers electrons to Complex IV.     - Complex IV (Cytochrome c oxidase): Pumps protons; reduces $O_2$ to $H_2O$.

• Mechanism: Electron flow establishes the proton motive force ($\Delta p$), consisting of an electrical gradient ($\Delta \psi$) and chemical gradient ($pH$ difference).

Oxidative Phosphorylation

• ATP Synthase ($F_0F_1$-ATPase):     - $F_0$ Unit: Membrane rotor; proton flow drives c-ring rotation.     - $F_1$ Unit: Catalytic head; converts ADP + Pi to ATP through rotary catalysis.

• Chemiosmotic Coupling: Protons return to the matrix only through ATP synthase, converting gradient energy to chemical energy.

• Respirasomes: Supercomplexes of ETC components that increase efficiency and reduce ROS leakage.

• Uncoupling: Uncoupling Protein 1 (UCP1) in brown adipose tissue dissipates the gradient to produce heat (non-shivering thermogenesis).

Biosynthetic Functions of Mitochondria

• Heme Biosynthesis: Requires succinyl-CoA from the TCA cycle; occurs partially in the matrix.

• Iron-Sulfur (Fe-S) Cluster Assembly: Crucial for ETC and nuclear genome stability (defects cause Friedreich ataxia).

• Steroidogenesis: Conversion of cholesterol to pregnenolone (requires StAR protein).

• Phospholid Metabolism: Synthesis of cardiolipin, phosphatidylethanolamine (PE), and phosphatidylglycerol (PG).

• Urea Cycle: Initial step (carbamoyl phosphate production) occurs in the mitochondria.

Reactive Oxygen Species (ROS) and Thermogenesis

• ROS Production: Major source is ETC Complexes I and III leaking electrons, forming superoxide ($O_2^{\cdot -}$), later converted to $H_2O_2$ and hydroxyl radicals ($\cdot OH$).

• Antioxidants: SOD2, glutathione peroxidase, and the thioredoxin system.

• Pathology: ROS causes oxidative damage to mtDNA, cardiolipin, and proteins, contributing to neurodegeneration (Alzheimer’s, Parkinson’s) and aging.

Calcium Homeostasis and MAMs

• Calcium Buffering: Mitochondria take up $Ca^{2+}$ via the MCU complex (Mitochondrial Calcium Uniporter) driven by $\Delta \psi$. Release occurs via NCLX (Na$^+$/Ca$^{2+}$ exchanger).

• Metabolic Activation: Matrix $Ca^{2+}$ activates Pyruvate Dehydrogenase, Isocitrate Dehydrogenase, and $\alpha$-ketoglutarate dehydrogenase to boost ATP.

• MAMs (Mitochondria-Associated Membranes): Regions where ER and mitochondria are linked (by MFN2, VAPB-PTPIP51). Facilities transfer of $Ca^{2+}$ and lipids.

Mitochondria in Apoptosis (Intrinsic Pathway)

• MOMP (Mitochondrial Outer Membrane Permeabilization): The "point of no return." BAX and BAK form pores in the OM.

• Factor Release: IMS releases Cytochrome c and Smac/DIABLO (inhibits IAPs).

• Apoptosome: Cytochrome c + Apaf-1 + procaspase-9. Activates executioner caspases-3 and -7.

• Dynamics: Fission and cristae remodeling (via OPA1) facilitate the mobilization/release of cytochrome c.

Mitochondrial Genome (mtDNA)

• Structure: Small circular DNA molecule, $\sim 16.6\,kb$ ($16,569\,bp$) in humans.

• Gene Content: 37 genes (13 protein-coding/OXPHOS subunits, 22 tRNAs, 2 rRNAs).

• Nucleoids: mtDNA is packed with TFAM protein into nucleoids.

• Replication/Transcription: Replicated by POLG ($DNA\,polymerase\,\gamma$) and transcribed by POLRMT.

• Unique Features: Maternal inheritance, no introns, compact arrangement ($93\%$ coding vs. $3\%$ in nuclear DNA), and high mutation rate.

Comparative Genomics: Nuclear vs. Mitochondrial DNA

• Size: Nuclear is $\sim 3.3 \times 10^9\,bp$; mtDNA is $16,569\,bp$.

• Molecules per cell: Nuclear is 23/46; mtDNA is several thousand (polyploidy).

• Gene Density: Nuclear is 1 per $40,000\,bp$; mtDNA is 1 per $450\,bp$.

• Codon Variations: In mtDNA, AUA codes for Methionine (not Isoleucine), TGA for Tryptophan (not Stop), and AGA/AGG act as stop codons.

• Replication: Nuclear follows strand-coupled S-phase; mtDNA follows strand-coupled and strand-displacement models, occurring throughout the cell cycle.

Heteroplasmy and Mitochondrial Diseases

• Heteroplasmy: Coexistence of mutant and wild-type mtDNA. Distribution varies by tissue (mitotic segregation).

• Threshold Effect: Disease occurs when mutant mtDNA exceeds a threshold ($60\u201390\%$).

• Affected Organs: Mainly brain (seizures), muscle (weakness), heart (cardiomyopathy), and eyes (optic neuropathy).

• Representative Disorders:     - MELAS: Encephalomyopathy, lactic acidosis, stroke-like episodes.     - MERRF: Myoclonic epilepsy with ragged red fibers.     - LHON: Leber hereditary optic neuropathy.     - Barth Syndrome: Cardiolipin remodeling defect (TAZ mutation).

Nuclear-Mitochondrial Cross-talk

• Signaling Hub: Mitochondria influence epigenetics via acetyl-CoA (sirtuins, HATs, HDACs), nutrient sensing (AMPK, mTOR), and DNA Damage Response (PARP, ATM).

• MAVS: Mitochondria participate in innate immune responses to viral infection.

• Disease Genes: 338 genes categorized (102 for OXPHOS, 102 for mtDNA homeostasis, 43 for dynamics/quality control, 40 for substrate metabolism, 41 for cofactors, 10 for toxins). Inheritance is mostly autosomal recessive (262 genes).